When pumping oil (or for that matter water or other fluids) from wells of a significant depth, typically a downhole pump is utilized wherein the pump is physically located deep within the well to pump the oil or fluid to the surface. In many such applications the downhole pump of choice is a screw or progressive cavity pump. Screw or progressive cavity pumps generally operate through the revolution of a pump rotor within a stator. A rotating pump rod extends from the surface to the pump to drive the rotor. A power supply, which would most commonly be comprised of a gas or diesel engine or an electric motor, provides the means to rotate the pump rod. The pump rod is normally mechanically connected to the drive motor such that rotation of the drive motor causes direct rotation of the pump rod, and hence the pump rotor. A series of seals are used to engage the rod at the point where it exits the top of the well to prevent downhole fluids from leaking into the environment.
One of the more significant problems encountered with a screw or progressive cavity pumping operation is the back spin that can occur when the pump drive mechanism is shut down or fails. When the drive motor stops there will normally exist a very large amount of stored energy in the pump rod, similar to a wound coil spring. That is, during operation in a typical well the pump rod can be torqued to the extent that it physically winds up like a spring. When the drive motor shuts down that stored energy is released through back spin of the rod. There may also exist a large head of fluid within the well placing significant back pressure upon the pump rotor. The pressure of this head of fluid may contribute to the back spin of the pump rod as the fluid drains back into the well, particularly where the fluid is of a high specific gravity or where the well is relatively deep.
Through the release of torque stored in the pump rod and the draining of a significant head of fluid into the well, when the drive motor is shut down there can be experienced a back spin of the pump rotor at very high speeds, in some instances approaching a few thousand rpm. Since the pump drive system is often directly connected to the rotor, the entire drive system will be subjected to back spin. Uncontrolled back spin of this nature can severely damage the drive mechanism and other production equipment. In some instances the back spin can be so severe as to result in equipment destruction. Where such destruction occurs at the surface of the well there is the possibility of personal injury and environmental contamination.
In order to combat potential back spin in screw or progressive cavity pumping applications, others have developed a variety of different clamping or braking devices that can be used to physically grasp the pump rod when the drive mechanism has shut down. Unfortunately, the effectiveness of such braking mechanisms is limited as they prevent back spin by merely preventing rotation of the pump rod and in so doing result in the imposition of high levels of torque upon the drive, the rod and the pump. High levels of torque are undesirable as they may cause damage to the drive system and/or the pump. In addition, when pumping is ultimately resumed, the pump drive system must be started "under load" putting further strain on the drive motor and drive system.